Paper cut: Better handling of "CoverageException: Can't add arcs to existing line data" ?

[PiDelport]

Is your feature request related to a problem? Please describe.

Previous closed issue, with comments: #476

Reproduction: In the process of adding coverage to an existing project's test suite, I first ran it without branch coverage, and then again after enabling branch coverage in .coveragerc, resulting in "CoverageException: Can't add arcs to existing line data".

I'm familiar enough with coverage to have guessed that it's a data file format problem, and deleted the .coverage file to fix it, but this won't be obvious to new or inexperienced users, and it probably represents a significant usability hurdle / paper cut bug (especially since it's most likely to occur when first adding coverage to a project in this way)

Describe the solution you'd like

One or more of:

1. A more descriptive and actionable error message: at least, something that would lead new / inexperienced users to clearing the coverage database, for example.
2. More generally, a mechanism to recognise when the coverage database format is not compatible with a coverage run's invocation options, so that this kind of case can be handled better in general?

Additional context

This is the relevant traceback (with coverage 4.5.3, invoked via nosetests --with-coverage):

Traceback (most recent call last):
  […]
  File "…/site-packages/coverage/control.py", line 807, in combine
    self.get_data()
  File "…/site-packages/coverage/control.py", line 833, in get_data
    if self.collector.save_data(self.data):
  File "…/site-packages/coverage/collector.py", line 404, in save_data
    covdata.add_arcs(abs_file_dict(self.data))
  File "…/site-packages/coverage/data.py", line 365, in add_arcs
    raise CoverageException("Can't add arcs to existing line data")
coverage.misc.CoverageException: Can't add arcs to existing line data

[nedbat]

I think you are right, this is definitely a place where the messaging could be made more relevant to the user's perspective rather than the code's perspective, so that the user understands what's happening.

[nedbat]

This is now released as part of coverage 5.5.